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NTRODUCTION

MAGNESIUM–rare earth (RE) alloys, such as WE43 and WE54, are recognized and used as high-performance lightweight constructional materials for sustained applications up to 250 °C. Nevertheless, they do not achieve the outstanding creep resistance of the thorium containing Mg alloys (ZH62, HZ32), which can operate at temperatures up to 350 °C.[1,2] Since thorium-containing Mg alloys are being phased out due to the slight radioactivity of Th, it was apparent that the only possibility to improve the high-temperature creep resistance beyond that of WE alloys was to turn to another Mg-RE system. The Mg-Sc system was promising for several reasons: (a) alloying with Sc increases the melting point of the MgSc solid solution (it is the only peritectic system among the Mg-RE alloys);[3] (b) the high melting point of Sc compared to other REs (1541 °C) signifies a lower diffusivity in Mg; (c) the density of Sc (2989 kg/m3) is significantly lower than other REs; and (d) alloying with other elements that form intermetallic compounds with Sc or complex precipitates with Sc and Mg might improve further the high-temperature properties. The development of highly creep-resistant magnesium alloys based on alloying with scandium encompassed an investigation of squeeze-cast binary Mg-Sc, ternary Mg-ScB.L. MORDIKE, Professor Emeritus, is with the Institute of Materials Engineering and Technology, TU Clausthal, 38678 Clausthal, Germany. ´ I. STULIKOVÁ and B. SMOLA, Associate Professors, are with the Faculty of Mathematics and Physics, Charles University, 121 16 Prague, Czech Republic, and all authors are with Zentrum für Funletions werestoffe, 38678 ClausthalZellerfeld, Germany. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A

Mn, quaternary Mg-Sc-Ce-Mn, Mg-Y-Sc-Mn, Mg-Gd-ScMn, Mg-Ce-Sc-Mn, and complex Mg-Y-Nd-Sc-Mn alloys. The results of the creep behavior of the alloys in the temperature range of 200 °C to 350 °C and correlated with the corresponding structure are summarized and discussed here. II. EXPERIMENTAL DETAILS Almost all alloys investigated were squeeze cast at ZfW gGmbH (Clausthal, Germany). The starting materials were metallic Sc, Gd, Ce, and Y stirred into molten Mg or MgMn (1.56 wt pct) master alloy (binary or ternary and quaternary alloys). Residual quantities of the MgSc15Mn1 alloy and Mg-Nd (12.0 wt pct, minimum, 11.5 wt pct Nd-MEL) master alloy were used for cast Mg-Y-Nd-Sc-Mn. Squeeze casting was carried out at an applied pressure of about 130 MPa in a protective atmosphere (Ar-SF6). Only MgSc15 and MgSc19 were sand cast and ingot cast, respectively, by MEL (Magnesium Elektron Ltd., Manchester, United Kingdom). Table I summarizes the composition of the cast alloys as determined by atomic absorption. Te